Scandium is extremely valuable as an additive for high-strength alloys and an electrode material for fuel cells. However, scandium has not yet been used widely due to the small production quantity and high cost thereof.
Meanwhile, a trace amount of scandium is contained in nickel oxide ore such as laterite ore and limonite ore. However, nickel oxide ore has not been used industrially as a raw material for nickel for many years because the grade of nickel in nickel oxide ore is low. Consequently, very few studies also have been conducted for a method of industrially recovering scandium from nickel oxide ore.
Nonetheless, in recent years, the high pressure acid leach (HPAL) process has been emerging as a practical method, in which nickel oxide ore is introduced into a pressure vessel along with sulfuric acid, and heated at a high temperature of 240° C. to 260° C. to allow solid-liquid separation into a nickel-containing leachate and a leach residue. In the HPAL process, a neutralizing agent is added to the leachate obtained to separate impurities, and then a sulfurizing agent is added to the resulting leachate from which impurities are separated out, allowing recovery of nickel as nickel sulfide. Subsequently, this nickel sulfide may be subjected to a known nickel refinement process to obtain electrolytic nickel and nickel salt compounds.
In the case of using the HPAL process as described above, scandium which has been contained in nickel oxide ore is contained in the leachate together with nickel (see Patent Document 1). Subsequently, a neutralizing agent is added to the leachate obtained through the HPAL process to separate impurities and then a sulfurizing agent is added to the leachate from which the impurities have been removed to recover nickel as nickel sulfide. Meanwhile, scandium cannot be separated by the above method but remains in the acidic solution after the addition of a sulfurizing agent. In this way, nickel can be effectively separated from scandium by using the HPAL process.
However, the content of scandium contained in nickel oxide ore is generally significantly low, thus the concentration of scandium contained in the acidic solution after the addition of a sulfurizing agent (also referred to as post-sulfuration liquid or barren liquor) in the above method is significantly low to be at mg/l level, and it is difficult to efficiently recover scandium directly from the acidic solution.
For this reason, a treatment for enriching scandium contained in the post-sulfuration liquid and separating the coexisting impurities at the same time is required. As a specific enriching means, for example, there is a method in which enriching is performed using a chelating resin (see Patent Document 2).
In the method disclosed in Patent Document 2, nickel and scandium are first selectively leached from nickel-containing oxide ore into an acidic aqueous solution in an oxidizing atmosphere at high temperature and high pressure to obtain an acidic solution, subsequently the pH of the acidic solution is adjusted to a range of 2 to 4, and then nickel is selectively precipitated and recovered as a sulfide using a sulfurizing agent. Next, the solution obtained after nickel recovery is brought into contact with a chelating resin to adsorb scandium to the chelating resin, the chelating resin is washed with a dilute acid, and then the chelating resin after being washed is brought into contact with a strong acid to elute scandium from the chelating resin.
Further, as a method of recovering scandium from the aforementioned acidic solution, the method of recovering scandium by means of solvent extraction has also been proposed (see Patent Documents 3 and 4).
In the method described in Patent Document 3, an organic solvent prepared by diluting 2-ethylhexylsulfonic acid-mono-2-ethylhexyl with kerosene is first added to a scandium-containing solution of an aqueous phase which contains at least one or more kinds of iron, aluminum, calcium, yttrium, manganese, chromium, or magnesium in addition to scandium, and the scandium component is extracted into the organic solvent. Subsequently, in order to separate yttrium, iron, manganese, chromium, magnesium, aluminum and calcium extracted into the organic solvent together with scandium, these are removed by adding an aqueous solution of hydrochloric acid to the organic solvent and performing scrubbing, and then an aqueous NaOH solution is added to the organic solvent to obtain a slurry containing Sc(OH)3 transformed from scandium remaining in the organic solvent, Sc(OH)3 obtained by filtering this slurry is dissolved with hydrochloric acid to obtain an aqueous solution of scandium chloride. Thereafter, oxalic acid is added to the resulting aqueous solution of scandium chloride to generate a precipitate of scandium oxalate, the precipitate is filtered to separate iron, manganese, chromium, magnesium, aluminum and calcium into the filtrate, and then the precipitate is calcined to obtain high purity scandium oxide.
Moreover, Patent Document 4 describes a method of selectively separating and recovering scandium from a scandium-containing supply liquid, the method including: bringing the scandium-containing supply liquid into contact with an extracting agent at a certain ratio in a batch process.
However, it cannot be said that purification can be easily performed in the case of treating actual nickel oxide ore even though various separation methods as described above are known. The leachate obtained by leaching nickel oxide ore with an acid contains impurities such as iron and aluminum at a much higher concentration than scandium together with scandium, and it is not easy to completely separate the impurities only by a method using a chelating resin and solvent extraction.
Furthermore, nickel oxide ore contains actinoid elements such as thorium in a trace amount in some cases. In this case, according to the method, in which a chelating resin and an organic solvent is used, disclosed in Patent Document 2 and Patent Document 3, it is difficult to efficiently separate scandium from actinoid elements since a number of actinoid elements such as thorium exhibit similar behavior to scandium.
It is required to decrease the concentration of actinoid elements to a concentration of, for example, less than 1 mg/l at the stage of a solution before obtaining a solid containing scandium in order to secure the properties of the product as well as to increase purity of scandium by separating impurities from scandium particularly in order to use recovered scandium in high-performance applications such as electrode materials for a fuel cell.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H03-173725
Patent Document 2: Japanese Unexamined Patent Application, Publication No. H09-194211
Patent Document 3: Japanese Unexamined Patent Application, Publication No. H09-291320
Patent Document 4: PCT International Publication No. WO2014/110216